Anchor

Details Anchor Anchor

2000-01-12

2001-06-05

Avila, Stephen (Department: 3617)

Ships

Anchor

With means driving the anchor into the sea bed

C114S301000

Reexamination Certificate

active

06240870

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a mooring device intended to be used for mooring vessels. More specifically, the present invention relates to a vertical-load mooring device intended to be used particularly for mooring vessels which are frequently moved between operating sites.
PRIOR ART
Since the beginning of offshore petroleum exploration, it has been necessary to use anchors to moor the floating units which are used, for example, for drilling production wells or subsequently for receiving and processing the oil produced.
The anchors originally used were designed to withstand only horizontal loading. To meet this requirement, it was therefore necessary for the mooring lines to extend from their point of connection with the anchors in the direction of the floating unit in an essentially horizontal configuration and then, gradually, to rise towards the floating unit to be moored, forming a catenary.
Consequently, each of the anchors was generally located at a very great distance from the floating unit, this being a distance, in the case of very deep waters, of several kilometers. With the expansion in offshore petroleum exploration and production, and also with wells being drilled in increasingly deeper waters, this catenary configuration of the mooring lines began to pose a number of problems.
One of these problems, probably the most serious, relates to the characteristic of the catenary configuration of the mooring lines requiring a very large area for mooring the floating units, especially in very deep waters, since there are situations in which interference might arise between the mooring lines of different units, which has an adverse effect on the precise location of the floating units. There may also be interference between mooring lines and flowlines for the production of the petroleum wells, particularly in regions where petroleum exploration is intense.
To solve this problem, vertical-load anchors were developed which, as their very name suggests, are capable of operating with a substantially vertical load in order to moor a floating unit, thereby substantially reducing the area required for mooring floating units.
U.S. Pat No. 5,540,175, U.S. Pat. No. 5,546,883, U.S. Pat. No. 5,546,884 and U.S. Pat. No. 5,640,921, the descriptions of which are incorporated herein by way of reference, describe anchors which are capable of receiving a vertical load, anchor flukes, and mooring methods which were devised to solve the above problems.
In the current state of the art there are also other anchors which are capable of operating with a vertical load and which solve the above problems, but which are not mentioned here as they are not relevant to the issue.
The major advantage of such anchors is their capacity to support the same load as a suction pile with a much smaller weight (an anchor is approximately 10% of the weight of a suction pile). In addition, they have the advantage that burying them is more rapid and much more economical.
Such anchors are usually installed with the aid of an anchor handling boat which is specially set up for this task, although other means may be considered. In order to be buried in the seabed, an anchor has to be deposited on the seabed in a predetermined position and then the anchor handling boat pulls on an installation line connecting the anchor to the anchor handling boat and, as a consequence, the anchor is dragged along the seabed. To make it possible for the anchor to sink into the ocean floor, an angle of attack is defined between (i) the line which is being pulled by the anchor handling boat and (ii) the anchor. This angle of attack defines the trajectory of the anchor within the ocean floor. It should be pointed out that the anchor fluke is configured in such a manner that, as the anchor is moved forward, the anchor buries itself in the ocean floor. Consequently, it is possible to obtain very deep burial over short distances of travel.
After the desired burial depth has been achieved, it is then necessary to alter the angle of attack to 90° in order to ensure maximum mooring performance, since, in this manner, the anchor will later operate as a plate subjected to a force which is perpendicular to the area of resistance.
The design and proper functioning of the device which allows the change in the angle of attack are two fundamental characteristics which determine the ease of operation of a vertical-load anchor. Basically, two different configurations are known for allowing the change in the angle of attack to take place, namely:
a first configuration, in which two lines depart from the anchor, the first dedicated only to the burial of the anchor in the ocean floor and the second intended to operate as a mooring line;
a second configuration, in which only one line extends from the anchor, this fulfilling both functions (burial and anchoring).
The first, two-line, configuration requires the use of two support vessels, one for burying the anchor in the ocean floor and the other for connecting the mooring line to the floating unit to be moored.
The second, single-line, configuration is obviously more economical since, in addition to requiring less equipment, it allows the anchor to be buried with the use of only one vessel. In this case, it is necessary to use special devices to change the angle of attack to 90°.
Devices which are known in the prior art for changing the angle of attack in the case of the single-line configuration use shearing pins. In this situation, the anchor's front mooring cables, which are the same length as the anchor's rear mooring cables, have their length temporarily reduced, by means of a device which uses the shearing pins, in order thereby to promote the angle of attack for full burial of the anchor.
The shearing pins are designed to break when the anchor has been buried to a predetermined burial depth. In other words, when the anchor has reached the planned burial depth, the load exerted by the anchor's front mooring cables on the shearing pins will cause the pins to break. Consequently, the front mooring cables will then be the same length as the rear mooring cables, and the anchor will be ready to be taken to its correct operating position, in accordance with techniques which are not described here as they are widely known in the prior art.
The shearing-pin concept is a solution which has already been tested successfully, although it presents a number of drawbacks. The first of these is the definition of the pin's breaking load, which depends on precise knowledge of the characteristics of the ocean floor. This type of knowledge involves having a special vessel available to gather samples from the ocean floor for analysis, with a view to determining more accurately the resistance of the ocean floor to burial of the anchor, in order to give the shearing pin the dimensions it requires for breaking when the anchor has reached the desired burial depth.
As mooring radii are large, the situation may arise in which a number of anchors in the same mooring system are located in regions where the ocean floor has different characteristics and, consequently, there may be differences in the loads required for the shearing pins of the various anchors to break, which requires a greater number of samples to be gathered, thereby making the mooring process more expensive.
The second problem relates to the fact that, if a shearing pin breaks before the anchor has reached the desired burial depth, it is necessary to remove the anchor from the ocean floor in order to repeat the burial operation since, once the pin has broken, it is no longer possible to attempt to reach the desired burial depth.
Another negative point relates to the fact that current removal systems require a precise procedure for the movement of the handling boat in order to prevent damage to the anchor, and this frequently requires the operation to be repeated, thereby making it more expensive.
OBJECTS AND SUMMARY OF THE INVENTION
It is a main object of the present invention to provide an anchor which may be buried in

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